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Infinite rows in QTableWidget?

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  • artwawA artwaw

    @Publicnamer since rowCount() returns an int there is your theoretical limit.

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    Publicnamer
    wrote on last edited by
    #4

    @artwaw I don't think you read my question.

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    • mrjjM Offline
      mrjjM Offline
      mrjj
      Lifetime Qt Champion
      wrote on last edited by mrjj
      #5

      Hi
      Normally a custom model is used instead
      https://doc.qt.io/qt-5/qtwidgets-itemviews-fetchmore-example.html

      key here is
      bool canFetchMore(const QModelIndex &parent) const override;
      void fetchMore(const QModelIndex &parent) override;

      Adding QTableWidget* on scrolling might not be a smooth experience.

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      • mrjjM mrjj

        Hi
        Normally a custom model is used instead
        https://doc.qt.io/qt-5/qtwidgets-itemviews-fetchmore-example.html

        key here is
        bool canFetchMore(const QModelIndex &parent) const override;
        void fetchMore(const QModelIndex &parent) override;

        Adding QTableWidget* on scrolling might not be a smooth experience.

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        Publicnamer
        wrote on last edited by
        #6

        @mrjj Is the QTableWidget efficient so that if there are millions of empty cells, there is not a million times additional use of RAM?

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        • P Publicnamer

          @mrjj Is the QTableWidget efficient so that if there are millions of empty cells, there is not a million times additional use of RAM?

          mrjjM Offline
          mrjjM Offline
          mrjj
          Lifetime Qt Champion
          wrote on last edited by
          #7

          @Publicnamer
          Hi
          Yes, cells that are not allocated returns a NULL ptr for item() so it wont waste ram as such but
          creating millions of items will take time.

          So for huge models, often a custom model is used where the internal data structure fits the demand for such a huge amount of rows/items.

          But what will work best depends on what you actually need to do.

          P artwawA 2 Replies Last reply
          1
          • mrjjM mrjj

            @Publicnamer
            Hi
            Yes, cells that are not allocated returns a NULL ptr for item() so it wont waste ram as such but
            creating millions of items will take time.

            So for huge models, often a custom model is used where the internal data structure fits the demand for such a huge amount of rows/items.

            But what will work best depends on what you actually need to do.

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            Publicnamer
            wrote on last edited by
            #8

            @mrjj Regarding the methods canFetchMore and fetchMore, I subclassed QTableWidget and added these methods,
            but I am finding they are never called. Also none of the cells that previously appeared now appear.
            I'm wondering if this is a slightly advanced topic and a more practical solution would be better.

            Is there a way to detect when the user has scrolled to the last row?

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            • JoeCFDJ JoeCFD

              there are scroll mode, overshooting policy and frame rate settings. From them, you maybe able to know the actual location of scrolling and can decide when more rows are added or replaced.

              Maybe create a few frames and restart from the beginning when the last frame reaches. Sure, you update the frames after they pass.

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              Publicnamer
              wrote on last edited by
              #9

              @JoeCFD Looking in the class documentation and parent classes' as well, I don't see an overshoot policy mentioned anywhere.

              JoeCFDJ 1 Reply Last reply
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              • SGaistS Offline
                SGaistS Offline
                SGaist
                Lifetime Qt Champion
                wrote on last edited by
                #10

                Hi,

                You can check the visualRect of the last index. If it's valid then it means the index is shown.

                Interested in AI ? www.idiap.ch
                Please read the Qt Code of Conduct - https://forum.qt.io/topic/113070/qt-code-of-conduct

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                • P Publicnamer

                  @JoeCFD Looking in the class documentation and parent classes' as well, I don't see an overshoot policy mentioned anywhere.

                  JoeCFDJ Offline
                  JoeCFDJ Offline
                  JoeCFD
                  wrote on last edited by JoeCFD
                  #11

                  @Publicnamer

                      QScrollerProperties properties = QScroller::scroller( your widget )->scrollerProperties();
                      QVariant overshoot_policy = QVariant::fromValue< QScrollerProperties::OvershootPolicy >( QScrollerProperties::OvershootAlwaysOff );
                      properties.setScrollMetric( QScrollerProperties::VerticalOvershootPolicy, overshoot_policy );
                      properties.setScrollMetric( QScrollerProperties::HorizontalOvershootPolicy, overshoot_policy );
                      QVariant frame_rates = QVariant::fromValue< QScrollerProperties::FrameRates >( QScrollerProperties::Fps60 );
                      properties.setScrollMetric( QScrollerProperties::FrameRate, frame_rates );
                      QScroller::scroller( your widget )->setScrollerProperties( properties );
                  
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                  • mrjjM mrjj

                    @Publicnamer
                    Hi
                    Yes, cells that are not allocated returns a NULL ptr for item() so it wont waste ram as such but
                    creating millions of items will take time.

                    So for huge models, often a custom model is used where the internal data structure fits the demand for such a huge amount of rows/items.

                    But what will work best depends on what you actually need to do.

                    artwawA Offline
                    artwawA Offline
                    artwaw
                    wrote on last edited by
                    #12

                    @Publicnamer out of curiosity - why bother with custom model when you can choose the path of least effort and employ QSqlTableModel (or derive something from QIdentityProxyModel) with Sqlite? With Qt you can instantiate in-memory db or, if during your project you discover that memory becomes an issue, quickly drop it from memory to disk?
                    Sqlite is lightning fast.
                    Sure, it requires a bit of thinking ahead (sorting, potential delegates for the table view) but in my experience all "convenience" item views are simply too much of a problem.

                    For more information please re-read.

                    Kind Regards,
                    Artur

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                    • JoeCFDJ JoeCFD

                      @Publicnamer

                          QScrollerProperties properties = QScroller::scroller( your widget )->scrollerProperties();
                          QVariant overshoot_policy = QVariant::fromValue< QScrollerProperties::OvershootPolicy >( QScrollerProperties::OvershootAlwaysOff );
                          properties.setScrollMetric( QScrollerProperties::VerticalOvershootPolicy, overshoot_policy );
                          properties.setScrollMetric( QScrollerProperties::HorizontalOvershootPolicy, overshoot_policy );
                          QVariant frame_rates = QVariant::fromValue< QScrollerProperties::FrameRates >( QScrollerProperties::Fps60 );
                          properties.setScrollMetric( QScrollerProperties::FrameRate, frame_rates );
                          QScroller::scroller( your widget )->setScrollerProperties( properties );
                      
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                      Publicnamer
                      wrote on last edited by
                      #13

                      @JoeCFD That seems to have no effect on my QTableWidget. I set the policy to OvershootAlwaysOn.
                      When I scroll using the mouse's thumbwheel is still stops at the defined last row.

                      Is there a signal that I can catch to detect when scrolling has stopped?
                      Thanks.

                      JoeCFDJ 1 Reply Last reply
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                      • P Publicnamer

                        @JoeCFD That seems to have no effect on my QTableWidget. I set the policy to OvershootAlwaysOn.
                        When I scroll using the mouse's thumbwheel is still stops at the defined last row.

                        Is there a signal that I can catch to detect when scrolling has stopped?
                        Thanks.

                        JoeCFDJ Offline
                        JoeCFDJ Offline
                        JoeCFD
                        wrote on last edited by JoeCFD
                        #14

                        @Publicnamer I am using it for tablewidget in my code to avoid overshooting. This piece of code works fine for me on Ubuntu.
                        https://www.qtcentre.org/threads/16937-QTableWidget-scrolling-to-bottom-of-the-table

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                        • SGaistS SGaist

                          Hi,

                          You can check the visualRect of the last index. If it's valid then it means the index is shown.

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                          Publicnamer
                          wrote on last edited by Publicnamer
                          #15

                          @SGaist said in Infinite rows in QTableWidget?:

                          You can check the visualRect of the last index. If it's valid then it means the index is shown.

                          OK yes I can listen for the vertical scrollbar's valueChanged signal and check for the visualRect there.

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                          • P Publicnamer

                            @SGaist said in Infinite rows in QTableWidget?:

                            You can check the visualRect of the last index. If it's valid then it means the index is shown.

                            OK yes I can listen for the vertical scrollbar's valueChanged signal and check for the visualRect there.

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                            Publicnamer
                            wrote on last edited by Publicnamer
                            #16

                            @Publicnamer Actually that doesn't work, because all rows have valid visualItemRect's.

                                QRect rect = mytable->visualItemRect (item);
                                if (rect.isValid()) { /* always true */ }
                            

                            The only solution I see is to use itemAt to find out what's at the bottom of the table.

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                            • P Publicnamer

                              Is there a way to tell QTableWidget to support infinite rows?

                              If not, how could I detect when the user scrolls to the last row, in order to automatically add more for the user?

                              JonBJ Offline
                              JonBJ Offline
                              JonB
                              wrote on last edited by JonB
                              #17

                              @Publicnamer
                              It depends just what you want, what is in your model, and whether you know how much is in the backing model.

                              Your question seems to relate to paging the data, or something based off that. Rather than looking at visual rects --- especially if they don't work --- the alternative is to look at the scrollbar. That reflects how far through the data the user is.

                              One possible implementation is given in Pagination in QtableWidget, @raven-worx's algorithmic outline there.

                              I have seen others (unfortunately can't find references) where the algorithm is to maintain the scrollbar height such that there is always a "gap" at the bottom which the user can scroll into, and that fires a fetch of more data and recalibration of the scrollbar, which still has a "gap" at the bottom. Think of when you use an application which produces large or endless output, like a web page receiving more and more data or showing photos where more and more are to come (e.g. some tumblr pages). The scrollbar always allows the user to scroll some more. @raven-worx's suggestion could be adapted to that.

                              If you Google for stuff involving QTableWidget or QTableView and pagination you may find some useful code/ideas.

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